1. Field of the Invention
This invention relates to improvements in travelling wave amplifiers (TWAs), and particularly millimeter wave travelling wave amplifiers.
2. Description of Related Art
Travelling waves are electromagnetic waves that travel along and are guided by a transmission line. If amplification of travelling waves is required, then it is desirable to use an amplifier which acts as a transmission line. Such travelling wave amplifiers (or distributed amplifiers) are known. These generally comprise two or more amplification modules connected together. The input and the output of each amplification module forms a so-called artificial transmission line. Each artificial transmission line includes series inductance components and shunt capacitance. The inductance components can be realised using high impedance transmission lines. The capacitance can be the parasitic capacitance associated with each amplification module. The characteristic impedance of the artificial transmission line is then given by Z0=Sqrt(L/C), i.e. is not frequency dependent, a desirable characteristic for TWAs. In addition, by careful choice of the values of the inductance and the capacitance, the characteristic impedance can be set at approximately 50Ω, the industry standard in TWA applications. The amplification modules are connected together by connecting together inductance components of each module. The inductance components used have high characteristic impedance, in the region of 100Ω.
The cut-off frequency of each amplification module of a TWA is related to the inductance and capacitance of the input and output artificial transmission lines thereof. The cut-off frequency is inversely proportional to the square root of the product of the inductance and capacitance. In order to increase the cut-off frequency, the inductance and capacitance can be reduced. To reduce the inductance t the length of the inductance components of each artificial transmission line may be reduced. However, this can result in a number of problems. For example, each amplification module has a finite size. If the length of the inductance components are reduced they may no longer be of sufficient size to connect the modules together. This may be particularly so in millimeter wave applications (i.e. high frequency applications), where the minimum separation between amplification modules may be longer than the required length of the inductance components used to connect them together.
Another problem with many TWAs arises because of unequal phase velocities in the input and the output artificial transmission lines of each amplification module. When it is desired to combine the amplified signal from each module such unequal phase velocities can result in some cancelling in the overall output signal. This problem can be overcome by making the electrical length of the inductance components in the output artificial transmission line of each module equal to the electrical length of the inductance components in the input artificial transmission line. However, increasing the length of the inductance components in the output artificial transmission line may have an adverse effect on the desired cut-off frequency and return loss.